The present invention relates to a gas turbine combustor and, more particularly to a gas turbine combustor, of a two-stage combustion system, which burns a gaseous fuel such as natural gas (LNG) producing relatively small amount of very NOx.
A method of reducing NOx in the gas turbine combustor is roughly divided into a wet-type method which uses water or water vapor, and a dry-type method which is based upon the improved combustion performance. In the former method a medium such as water is employed, with the resulting water vapor decreasing turbine efficiency. The dry-type method of reducing Nox is superior to the wet-type method, however, since dry-type method is to sustain combustion with a fall lean mixture at a low uniform temperature, carbon monoxide is generated in large amounts though only small amounts of NOx are generated.
During combustion, in general, formation of NOx is dominated by a combustion gas of a local high-temperature portion (higher than 1800.degree. C.) in the combustion region. NOx is formed mainly by two conditions, namely the oxidation of nitrogen contained in the uncombusted exhaust and the oxidiation of nitrogen contained in the combustion air. These two conditions will hereafter be called the thermal NOx and the fuel NOx. The thermal NOx is largely dependent upon the oxygen concentration and the reaction time, which, in turn, are affected considerably by the gas temperature. Therefore, combustion can be sustained while effectively reducing the formation of NOx if a uniform temperature lower than 1500.degree. C. is maintained without permitting the high-temperature regions to occur in the combustion.
To reduce the formation of NOx in the gas turbine, the lean diffusion combustion method has heretofore been most advantageously employed, since a gas turbine combustor permits a relatively large air flow rate with respect to the fuel flow rate, and it makes it possible to control the distribution of air in the combustion chamber to some extent. The chief concern is that combustion is performed over a low uniform temperature range, by reducing combustion temperature, facilitating mixing, and reducing time during which NOx is formed.
A conventional technique for realizing the above-mentioned combustion has been disclosed, for example in Japanese Patent Publicaiton No. 20122/1980, in which a plurality of fuel nozzles are annularly arranged in an annular combustion chamber, and the air and water vapor are introduced from the downstream side of an inner cylinder installed coaxially of the combustion chamber. The combustor employs a combustion method in which the fuel is supplied into the combustion chamber and dispersed over the cross section thereof, so as to make the combustion temperature uniform and to decrease gas temperature downstream of the combustion chamber. Further, flame stabilizers of the type disclosed, for example, in Japanese Patent Laid Open Application No. 202431/1982 consist of swirlers installed around the fuel nozzles for stabillizer the combustion flame in the region of whirling stream formed by whirling air. During combustion, however, extremely hot gases are present in the region of the whirling stream in order to maintain and stabilize the flame near the fuel nozzles, thereby making it difficult to reduce NOx. In the flame stabilizer having air whirling vanes, a relatively high air flow velocity (V&gt;30 m/s) is necessary to function within its effective range where the Reynolds number Re is greater than 10.sup.5. Further, as the flame is reduced in length, combustion is likely to take place most rapidly near the fuel nozzles. Moreover, an intense flame stabilization at a localized high-temperature portion in the region of whirling flow which is 1 to 2 times wider than the diameter of the flame stabilizer, induces the formation of NOx. Therefore, even if a plurality of fuel nozzles having a conventional flame stabilizer are provided, they are unlikely to greatly reduce the formation of NOx. Particularly for combustion in which NOx is formed in small amounts, it is essential to provide a flame stabilizing mechanism that effectively reduces the rate of NOx formation. The mode of combustion is greatly affected by the flame-stabilizing characteristics.
A combustor employing the two-stage combustion system has been disclosed, for example, in Japanese Patent Laid-Open No. 41524/1982. In this combustor, a pre-mixture gas of fuel and air is introduced into a first-stage (head) combustion chamber where combustion is effected by a single nozzle. Then, fuel and air are simultaneously supplied via air holes into a second-stage (rear) combustion chamber on the downstream side, in order to sustain low-temperature combustion with a lean mixture so that NOx is formed in reduced amounts.
However, according to the method in which a combustion flame is formed in a distributed manner by a single nozzle in the head combustion chamber, and the fuel in the second stage is introduced downstream, it is difficult to limit the formation of NOx. That is, formation of NOx can be suppressed in the combustion of the second stage by introducing fuel at the second stage. In the combustion taking place in a distributed manner in the first stage, however, hot spots are formed over wide areas, making it difficult to suppress the formation of NOx. Furthermore, the single nozzle which exists on the axis of the combustion chamber makes it difficult to properly mix the fuel with the air stream that flows from the side walls of the combustion chamber, giving rise to the formation of hot spots. Thus, with the conventional combustor having a single fuel injection nozzle at the head of the combustion chamber, it is difficult to greatly limit the formation of NOx. Even with the two-stage combustor as described above, it is essential to limit the formation of NOx in the first stage and in the second stage, in order to strictly limit the total formation of NOx. In the conventional technique having a single fuel nozzle on the axis of the head portion, however, it is not possible to strictly limit the formation of NOx.
Further, even if the above-mentioned multi-fuel nozzles with the conventional flame stabilizers are employed for first stage combustion in place of the above-mentioned single fuel nozzle, the formation of NOx is not greatly reduced in amounts. The flame generated by the multi-fuel nozzles is too firmly stabilized to prevent the formation of local high temperature portions. NOx formation takes place near the nozzles, and the produced NOx is reduced in the second stage combustion.